1. Field of the Invention
The present invention relates to a plasma display device employing a plasma display panel (hereinafter referred to as a PDP). In particular, the present invention is useful for improving luminous efficacy of the PDP and driving the PDP stably.
2. Description of the Prior Art
Recently, plasma TV (PDP-TV) receivers, a kind of plasma display devices employing the plasma display panel (PDP), have been spreading rapidly in the market for large-screen TV receivers.
FIG. 14 is an exploded perspective view illustrating an example of a conventional ac surface-discharge type PDP of a three-electrode structure.
In the ac surface-discharge type PDP shown FIG. 14, a discharge space 63 is formed between a pair of opposing glass substrates, a front substrate 51 and a rear substrate 58. Usually, the discharge space 33 is filled with a discharge gas at several hundreds Torrs or more. As the discharge gas, usually He, Ne, Xe, and Ar are used either alone or in combination with one or more of the others.
Disposed on the lower surface of the front substrate 51 serving as a display screen are a plurality of sustain electrode pairs (also called sustain-discharge electrode pairs) for formation of sustain discharge mainly for light emission for forming a display. Each of these sustain electrode pairs is composed of an X electrode and a Y electrode.
Usually, each of the X and Y electrodes is made of a combination of a transparent electrode and an opaque electrode for supplementing conductivity of the transparent electrode. The X electrodes 64-1, 64-2, . . . are comprised of transparent X electrodes 52-1, 52-2, . . . and corresponding opaque X bus electrodes 54-1, 54-2, . . . , respectively, and the Y electrodes 65-1, 65-2, . . . , are comprised of transparent Y electrodes 53-1, 53-2, . . . and corresponding opaque Y bus electrodes 55-1, 55-2, . . . , respectively. It is often that the X electrodes are used as a common electrode and the Y electrodes are used as independent electrodes.
Usually, a discharge gap Ldg between the X and Y electrodes in one discharge cell are designed to be small such that a discharge start voltage is not excessively high, and a spacing Lng between an X electrode of one of two adjacent discharge cells and a Y electrode of the other of the two adjacent discharge cells is designed to be large such that unwanted discharge is prevented from occurring between two adjacent cells.
The X and Y sustain electrodes are covered with a front dielectric 56 which, in turn, is covered with a protective film 57 made of material such as magnesium oxide (MgO). The MgO protects the front dielectric 56 and lowers a discharge start voltage because of its high sputtering resistance and high secondary electron emission yield.
Address electrodes (also called write electrodes, address-discharge electrodes, or A electrodes) 59 for generating an address discharge (also called a write discharge) are arranged on the upper surface of the rear substrate 58 in a direction perpendicular to the sustain electrodes (the X and Y electrodes). The address electrodes 59 are covered with a rear dielectric 60, and barrier ribs 61 are disposed between the address electrodes 59 on the rear dielectric 60. Phosphors 62 are coated in cavities formed by the wall surfaces of the barrier ribs 61 and the upper surface of the rear dielectric 60.
In this configuration, an intersection of a sustain electrode pair with an address electrode corresponds to one discharge cell, and the discharge cells are arranged in a two-dimensional fashion. In a color PDP, a trio of three kinds of discharge cells coated with red, green and blue phosphors, respectively, forms one pixel.
FIG. 15 and FIG. 16 are cross-sectional views of one discharge cell shown in FIG. 14 viewed in the directions of the arrows D1 and D2, respectively. In FIG. 16, the boundary of the cell is approximately represented by broken lines. In FIG. 16, reference numeral 66 denote electrons, 67 is a positive ion, 68 is a positive wall charge, and 69 are negative wall discharges.
Next operation of the PDP of this example will be explained.
The principle of generation of light by the PDP is such that discharge is started by a voltage pulse applied between the X and Y electrodes, and then ultraviolet rays generated by excited discharge gases are converted into visible light by the phosphor.
FIG. 17 is a block diagram illustrating a basic configuration of a plasma display device. The PDP (also called the plasma display panel or the panel) 91 is incorporated into the plasma display device 100. The PDP 91 is connected to a driving circuit 98 comprised of an X electrode driving circuit 95, a Y electrode driving circuit 96 and an address electrode driving circuit 97 for supplying voltages to the X, Y and address electrodes, via an X electrode terminal portion 92, a Y electrode terminal portion 93 and an address electrode terminal portion 94 which serve as connecting portions between the electrodes within the panel and external circuits, respectively. The driving circuit 98 receives video signals for a display image from a video signal source 99, converts the signals into driving voltages, and then supplies them to respective electrodes of the PDP 91.
FIGS. 18A-18C illustrate a concrete example of driving voltages in a case where the ADS (Address Display-Period Separation) system is employed for displaying gray scales.
FIG. 18A is a time chart illustrating driving voltages during one TV field required for displaying one picture on the PDP shown in FIG. 14. FIG. 18B illustrates waveforms of voltages applied to the address electrode 59, the X electrode 64 and the Y electrode 65 during the address period (also called the address-discharge period, or the write-discharge period) 80 shown in FIG. 18A. The X and Y electrodes are called the sustain electrodes, respectively, and they are referred to collectively as the sustain electrode pair.
FIG. 18C illustrates sustain pulse voltages (also called the sustain-electrode pulse driving voltages or the sustain discharge voltages) applied between all the X electrodes and all the Y electrodes, which are the sustain electrodes, simultaneously, and a voltage (an address voltage) applied to the address electrodes, during a sustain period (also called a sustain-discharge period, or a light-emission display period) 81 shown in FIG. 18A.
Portion I of FIG. 18A illustrates that one TV field 70 is divided into sub-fields 71 to 78 each having different plural numbers of light emission from one another. Gray scales are generated by a combination of one or more selected from among the sub-fields 71 to 78.
For example, in a case where each of the eight sub-fields is provided with a luminance weighted by a different weighting factor based upon the binary system, each of three primary-color emitting discharge cells provides 28 (256) gray scale levels of luminance, and the PDP is capable of producing about 16.78 millions of different colors.
Portion II of FIG. 18A illustrates that each sub-field comprises a reset period (also called a reset-discharge period) 79 for resetting the discharge cells to an initial state, an address period (also called an address-discharge period, or a write-discharge period) 80 for addressing discharge cells to be lighted and made luminescent, and a sustain period (also called a sustain-discharge period, or a light-emitting display period) 81.
FIG. 18B illustrates waveforms of voltages applied to the address electrode 59, the X electrode 64 and the Y electrode 65 during the address period 80 shown in FIG. 18A, and the waveforms are called the sustain pulse voltage waveforms. A waveform (an A waveform) 82 represents a voltage V0 (V) applied to one of the address electrodes 59 during the address period 80, a waveform (an X waveform) 83 represents a voltage V1 (V) applied to the X electrode 64, and waveforms (Y waveforms) 84 and 85 represent voltages V21 (V) and V22 (V) applied to ith and (i+1)th Y electrodes 65.
As shown in FIG. 18B, when a scan pulse 86 is applied to the ith Y electrode 65, in a cell located at an intersection of the ith Y electrode with the address electrode 59 supplied with the voltage V0, initially an address discharge occurs between the Y electrode and the address electrode, and then an address discharge occurs between the Y electrode and the X electrode. No address discharges occur at cells located at intersections of the Y electrodes with the address electrode 59 at ground potential. This applies to a case where a scan pulse 87 is applied to the (i+1)th Y electrode.
As shown in FIG. 16, in the cell where the address discharge has occurred, charges (wall discharges) are generated by the discharges on the surface of the dielectric film 56 and the protective film 57 covering the X and Y electrodes, and consequently, a wall voltage Vw (V) is produced between the X and Y electrodes. In FIG. 16, reference numeral 66 denote electrons, 67 is a positive ion, 68 is a positive wall charge, and 69 are negative wall charges. Occurrence of sustain discharge during the succeeding sustain period 81 depends upon the presence of this wall charge.
FIG. 18C illustrates sustain pulse voltages applied between all the X electrodes and all the Y electrodes which serve as the sustain electrodes simultaneously during the sustain period 81 shown in FIG. 18A.
The X electrodes are supplied with a sustain pulse voltage of a voltage waveform 88, the Y electrodes are supplied with a sustain pulse voltage of a voltage waveform 89, and the magnitude of the voltages of the waveforms 88 and 89 is V3 (V). The address electrode 59 is supplied with a driving voltage of a voltage waveform 90 which is kept at a fixed voltage V4 during the sustain period 81. The voltage V4 may be selected to be ground potential.
The sustain pulse voltage of the magnitude V3 is applied alternately to the X electrode and the Y electrode, and as a result reversal of the polarity of the voltage between the X and Y electrodes is repeated. The magnitude V3 is selected such that the presence and absence of the wall voltage generated by the address discharge correspond to the presence and absence of the sustain discharge, respectively.
In the discharge cell where the address discharge has occurred, discharge is started by the first sustain voltage pulse applied to one of the X and Y electrodes, and the discharge continues until wall charges of the opposite polarity accumulate to some extent. The wall voltage accumulated due to this discharge serves to reinforce the second voltage pulse applied to the other of the X and Y electrodes, and then discharge is started again. The above is repeated by the third, fourth and succeeding pulses.
In this way, in the discharge cell where the address discharge has occurred, sustain discharges occur between the X and Y electrodes the number of times equal to the number of the applied voltage pulses and thereby emit light. On the other hand, in the discharge cells where the address discharge has not occurred, the discharge cells do not emit light. The above are the basic configuration of the conventional plasma display device and a conventional driving method thereof.
The following are some principal techniques for improving the luminous efficacy in the plasma display devices and driving the plasma display devices stably.
(1) Japanese Patent Application Laid-Open No. 2002-72959 (laid open on Mar. 12, 2002) and Japanese Patent Application Laid-Open No. 2002-108273 (laid open on Apr. 10, 2002)
If a sustain voltage is lowered to reduce electric power consumed for light emission, i.e., to improve luminous efficacy, the amount of wall charges accumulated after light-emitting discharge is reduced, and as a result the sustain discharge is not maintained because the discharge voltage is not exceeded even when the subsequent sustain voltage is applied. Consequently, the light-emitting discharge is discontinued, and therefore the quality of displayed images are severely degraded. To solve this problem, in the above prior art (1), after lighting discharge cells by applying conventional sustain voltages, by increasing an absolute value of a voltage difference between the sustain electrode pair, the stable sustain discharge is produced when the sustain voltages are lowered to improve the luminous efficacy. However, there is a problem in that the luminance become lower than in the case of the conventional driving method, because the discharge is produced at lower voltages.
(2) Japanese Patent Application Laid-Open No. 2002-132215 (laid open on May 9, 2002)
In the conventional driving method and the above prior art (1), a discharge cell is made to generate a discharge only once for one sustain pulse, and discontinues discharging until the subsequent sustain pulse is applied. In the initial discharge, a current sufficient for the discharge is supplied, but the amount of produced ultraviolet rays saturates as the discharge current is increased, further, the intensity of visible light also saturates as the amount of the ultraviolet rays is increased, and therefore luminance hardly increases as the discharge current is increased. Further, if the discharge cell is driven at a current small enough to prevent saturation of luminance, the discharge itself becomes unstable, and consequently, the stable discharges cannot be repeated. The PDP needs to vary a lighted-discharge-cell ratio (an display-image-forming discharge cell ratio or a load factor) according to various images to be displayed on the PDP, and hence the required discharge currents also vary. Consequently, if the discharge cells are driven at lower current levels, the more unstable the discharges become.
The above prior art (2) applies a two-level voltage to the sustain electrodes such that initially a first discharge occurs and then a second discharge occurs, for the purpose of repeating the discharges stably and improving the luminous efficacy at the same time regardless of variations in the lighted-discharge-cell ratio. Further, the prior art (2) also varies timing of succeeding rise of the sustain pulses or the repetition periods of the sustain pulses according to the lighted-discharge-cell ratio of each of the sub-fields, and increases or decreases finely the number of the sustain pulses to retain continuity between luminances before and after the changeover of the sustain pulse waveforms according to the lighted-discharge-cell ratio. The first discharge utilizes an LC resonance of a panel capacitance Cp and an inductance Lr of a coil included in an electric power recovery circuit for recovering the capacitive current from the PDP into a capacitor and then releasing the capacitive current. That is to say, the first discharge occurs in a process in which the LC resonance causes the voltage to rise to its maximum and then to fall from its maximum to its minimum. In the process for the voltage to fall from its maximum to its minimum, at an instant when the first discharge starts to weaken, the saturation of the amount of the produced ultraviolet rays starts to be decreased by the limitation on the current, and thereafter, since the degree of saturation of the amount of produced ultraviolet rays for increasing discharge current is decreased, the luminous efficacy is improved. However, since the coil of the electric power recovery circuit is utilized, a complicated measure which increases or decreases finely the number of the sustain pulses was required to retain continuity between luminances before and after the changeover of the sustain pulse waveforms according to the lighted-discharge-cell ratio of each of the sub-fields.